DESCRIPTION: TRH and 5HT are colocalized in the projections of caudal raphe nuclei onto dorsal vagal complex (DVC) nuclei. TRH and 5HT synergism appears to be involved in the vagal control of gastric function. Microinjections of selective 5HT antagonists will be performed in DVC in order to identify the 5HT receptor subtype involved in the synergistic response. Serotonin release in the dorsal medulla of fed and fasting animals will be compared to document that serotonin release by caudal raphe nuclei projections plays a key role in enhanced acid secretion and motility after a meal. A possible role of excitatory amino acids and substance P as neurotransmitters in the vagal afferents which mediate gastric stimulatory response after a meal will be examined with the use of the agonists and the antagonists of substance P and glutamate. Studies will also be performed to identify DVC neurons (using extracellular recordings) which are activated by gastric distension. Effects and synergistic interactions of TRH and 5HT microinjection directly onto solitary nucleus or DVC nuclei on the "gastric responsive DVC neurons" will be examined.